In a manufacturing process of a semiconductor device or a flat panel display (FPD), a plasma processing apparatus for performing etching, deposition, oxidation, sputtering or the like by using a plasma is widely used. In the plasma processing apparatus, in order to use a RF power for plasma generation, a RF power of a predetermined frequency (generally, 13.56 MHz or above) is supplied from a RF power supply unit to a RF electrode (or an antenna) provided inside or outside a chamber. Further, in order to freely control energy of ions incident to a target substrate from a plasma, a RF power of a predetermined frequency (generally, 13.56 MHz or less) is supplied from the RF power supply unit to a RF electrode of a mounting table for mounting thereon the substrate.
The RF power supply unit includes a RF power supply for outputting a RF power and a matching unit for matching an impedance of the RF power side and an impedance of a load side (electrode, plasma and chamber). The RF power supply and a transmission cable are designed to have an output resistance of about 50Ω, and the impedance in the matching unit is set or controlled such that the impedance of the load side including the matching circuit becomes about 50Ω, i.e., such that the power of the reflected wave becomes minimum.
In general, the matching unit used in the plasma processing apparatus includes a plurality of variable reactance elements, and is configured as an automatic matching unit capable of variably controlling the load impedance by selecting impedance positions or values of the variable reactance elements by a stepping motor or the like.
If the impedance of the plasma load changes due to a pressure change or the like during the plasma processing, the automatic matching unit automatically corrects the load impedance to a matching point (50Ω) by variably controlling the impedance positions of the variable reactance elements. In order to perform the automatic matching operation, the automatic matching unit is provided with a circuit for measuring a load impedance, a controller that variably controls an impedance position of each variable reactance element by a stepping motor to match the measured value of the load impedance to the matching point (50Ω), and the like.
In general, the automatic matching unit includes two variable capacitors serving as the variable reactance elements in the matching circuit, the variable capacitors being respectively connected in parallel and in series to the load with respect to the RF power supply. Here, the electrostatic capacitance of the first variable capacitor connected to the load in parallel mainly operates to variably control the absolute value of the load impedance. Meanwhile, the electrostatic capacitance of the second variable capacitor connected to the load in series mainly operates to variably control the phase of the load impedance (phase difference between RF voltage and RF current).
A conventional typical automatic matching unit obtains an absolute error and a phase error by comparing a measured phase and a measured absolute value of a load impedance obtained from an impedance measuring circuit with matching point values, i.e., a reference absolute value and a reference phase. Further, the conventional automatic matching unit varies the electrostatic capacitance (capacitance position) of the first variable capacitor such that the absolute error becomes close to zero and also varies the electrostatic capacitance (capacitance position) of the second variable capacitor such that the phase error becomes close to zero (see, e.g., Japanese Patent Application Publication No. H10-209789).
In the plasma processing apparatus, the impedance of the plasma load changes dynamically and indefinitely due to a pressure change in the chamber or the like. Therefore, the automatic matching unit needs to perform an automatic matching operation capable of responding to changes in the load impedance rapidly and accurately.
However, in the conventional automatic matching unit for orthogonally and variably controlling the first and the second capacitor such that the absolute value error and the phase error of the impedance become close to zero, the variable capacitors are separately operated to control the load impedance and, thus, it is difficult to reliably and rapidly obtain convergence to the matching point.
In other words, actually, the first variable capacitor affects the phase as well as the absolute value of the load impedance, and the second variable capacitor affects the absolute value as well as the phase of the load impedance. Therefore, if the capacitance position of the first variable capacitor is varied such that the absolute value error becomes close to zero, the operating point of the load impedance becomes close to the matching point in view of the absolute value but becomes distant from the matching point in view of the phase. Meanwhile, if the capacitance position of the second capacitor is varied such that the phase error becomes close to zero, the operating point of the load impedance becomes close to the matching point in view of the phase but becomes distant from the matching point in view of the absolute value. The above-described orthogonal automatic matching is not based on the operations of the variable capacitors and does not have an algorithm for constantly and stably converging the operating point close to the matching point. Therefore, hunting occurs, or a long period of time is required to complete the matching.